1. Field of the Invention
The present invention relates to a centrifugal multiblade fan in which many blades are arranged around a rotatable shaft, and the fan is suitably used for a blower in an air conditioning system for a vehicle.
2. Description of Related Art
Conventionally, this kind of centrifugal multiblade fan with a front edge of its blade being tapered is described in JP-A-2000-009083 and JP-A-2006-125229. “The front edge of the blade being tapered” means that the centrifugal multiblade fan is a tapered-type fan with an inner diameter of the fan on its side shroud side (suction side) being larger than on its main shroud side (opposite side from the suction side).
Specifically, in the above-described conventional technologies, by gradually making shorter a leading edge of a camber line from the main-shroud side toward the side-shroud side, an upper front edge end shape viewed from a side surface is made a generally circular arc or generally elliptical.
As an effect of the tapered-type fan, the following is described in JP-A-2000-009083. Inflow resistance can be reduced since the inner diameter is extended in a side-shroud side region serving as an inflow port, whereas on the main-shroud side serving as a mainstream of the flow, an air blowing effect is effectively produced by taking advantage of a long blade chord.
As the effect of the tapered-type fan, the following is described in JP-A-2006-125229. In a region on a side-shroud side serving as a suction part, the suction part is made large and air capacity performance thereby improves; and the distance to a blade front edge is made large to attenuate a turbulence and noise reduction is thereby achieved. On the other hand, in the other regions, static pressure is improved because chord length is long as usual.
However, in the tapered-type fan of the above conventional technologies, on the side-shroud side, exfoliation at the blade front edge is easily caused, and performance degradation is thereby caused. This problem will be described below.
FIGS. 8A to 8C are diagrams illustrating problems of these conventional technologies.
Angles β1′ and β2′ in FIGS. 8B and 8C indicate inlet angles at the respective cross sections. The inlet angle is an angle between a tangential line of the positive pressure surface 1215 at a corner part 1217 on a positive pressure surface 1215-side; and a tangential line of a blade row line (alternate long and two short dashes line in FIGS. 8B and 8C) at the corner part 1217 on the positive pressure surface 1215-side, on respective cross sections of blades 121 (cross-sectional surface when the blade 121 is cut in a direction perpendicular to a rotatable shaft). The positive pressure surface 1215 is a surface of the blade 121 on a rotational direction R′-side, and a negative pressure surface 1216 is a surface on the opposite side from the rotational direction R′.
As is evident from FIGS. 8B and 8C, an inlet angle β2′ on a cross section taken along a line VIIIC-VIIIC on a side shroud 122-side is much larger than an inlet angle β1′ on a cross section taken along a line VIIIB-VIIIB on a main shroud 123-side. More specifically, in this comparative example, a front end of a camber line is made shorter toward the side shroud 122. Accordingly, directions of the front ends of the camber lines are significantly different between the side shroud 122-side and the main shroud 123-side. As a result, the inlet angles are also significantly different between the side shroud 122-side and the main shroud 123-side.
Therefore, in the centrifugal multiblade fan; as indicated by arrows in FIG. 8A, a change of an air flowing direction (change from a rotation axis direction to a radial direction) is comparatively gradual on the main shroud 123-side, whereas the change of the air flowing direction is rapid on the side shroud 122-side. Accordingly, inflow velocity on the side shroud 122-side is slower than on the main shroud 123-side. Moreover, a peripheral speed at a blade front edge is greater on the side shroud 122-side having a larger inner diameter than on the main shroud 123-side having a smaller inner diameter.
Thus, to limit the exfoliation at the blade front edge, it is desirable that the inlet angle should be made smaller from the main shroud 123-side toward the side shroud 122-side. However, in the above-described comparative example, contrarily, the inlet angle β2′ on the side shroud 122-side is larger than the inlet angle β1′ on the main shroud 123-side. Accordingly, discrepancy between an inflow condition (inflow velocity) and the inlet angle is made significant on the side shroud 122-side. Hence, the exfoliation at the blade front edge is caused, and eventually, performance degradation is caused.